Microwave filters



May 15, 1962 FIG.

FIG. 3

H. J. RIBLET MICROWAVE FILTERS Filed Aug. l5, 1958 FI G. 4

T JNVENTOR. HENRY J. RIBLET ATTORNEY United States Patent O 3,035,236 MICROWAVE FILTERS Henry J. Riblet, 35 Edmunds Road, Wellesley, Mass. Filed Aug. 15, 1958, Ser. No. 755,304 8 Claims. (Cl. S33-73) The present invention relates in general to direct coupled or quarter-wave coupled microwave filters and more particularly concerns novel high-Q narrow-band microwave filters capable of transmitting selected spectral components of incident microwave energy at very high power levels while minimizing the response of the filtering sections to frequencies outside the narrow band.

lt is well known in the microwave filter art that many of the techniques developed in connection with the syn- Ithesis of lumped parameter linear passive networks may be adapted for synthesizing microwave filters formed of a cascade of large, generally lossless, reflecting elements regularly spaced in a uniform waveguide. General synthesis procedures for quarter-wave coupled and directcoupled filters are set forth in Microwave Transmission Circuits, vol. 9 of the M.I.T. Radiation Laboratory Series, pp. 661-706. For both types of filters, synthesis procedures are described based on the use of a ladder network prototype having a prescribed insertion loss function. A closely related synthesis procedure for the design of direct coupled filters is also disclosed in a paper by S. B. Cohn entitled Direct-Coupled Resonator Filters in the Proceedings of the I.R.E., vol. 45, pp. 187-196, for February 1957.

Generally, prior high-Q Ifilters have consisted of a number of reflecting elements spaced within the waveguide by one-fourth or one-half the guide wavelength of microwave energy at the filter center frequency. While the desired filter selectivity is readily obtained with such structures, at high power levels the potentials developed in the waveguide portions between the reflecting elements may exceed the breakdown potential and arcing may occur.

Accordingly, the present invention contemplates and has as a primary object materially increasing the power handling capabilities of high-Q waveguide filters while still providing the desired degree of selectivity.

Still another object of the invention is to provide high-Q waveguide filters in -accordance with the preceding object of the minimum size required to selectively transmit a predetermined segment of the frequency spectrum of input microwave energy at a given power level.

According to the invention, the power handling capabilities are increased by spacing the reflecting elements an integral number of half wavelengths apart, at least two adjacent reflecting elements being no less than a wavelength apart. Space is conserved by arranging the spacing between adjacent elements so that the maximum potential developed in each cavity formed between adjacent refiecting elements is just under the breakdown potential of the cavity in View of the power level and bandwidth of incident microwave energy.

Other features, objects and advantages of the invention will be better understood from the following specification when read in connection with the accompanying drawing in which:

FIG. 1 is a lengthwise sectional view through a Waveguide filter section constructed according to Vthe invention;

FIG. 2 is a view along section 2 2 of FIG. l to better illustrate a typical reflecting element;

FIG. 3 is a lengthwise sectional View through a representative embodiment of a waveguide filter constructed according to the invention;

FIG. 4 is a graphical representation of the square of the maximum voltage as a function of frequency in the different sections of the filter of FIG. 3 and 3,035,236 Patented May 15, 1962 FIG. 5 is a lengthwise sectional view through a waveguide filter having the selectivity of the filter of FIG. 3 and capable of handling high power levels, but more compactly arranged.

With reference now to the drawing and more particularly FIG. l thereof, there is illustrated a lengthwise sectional view through a filter section constructed according to the invention. Since the arrangement of irises within a rectangular waveguide is well-known, sectional views are employed to best illustrate the principles of the invention by showing the spacing between adjacent refiecting elements within the waveguide. Thus, the waveguide section 11 may typically be a rectangular waveguide dimensioned to support the propagation of microwave energy of the frequencies selectively transmitted by the filter. Each end of the section 11 includes a reecting element such as inductive irises 112 and 13. It is important to note that the separation between irises 12 and 13 is an integral multiple of substantially one half the guide wavelength of microwave energy at the center frequency of the ulter formed by section 11 cascaded with other sections. While 4the other sections may be any integral multiple of half this guide wavelength, section 11 is two or more times the half guide wavelength.

The irises 12 and 13 are characterized by coefficients r2 and r1, respectively, which relate current and voltage on the load and source side of each iris. These relations are set forth in detail in the discussion below concerning the mode of operation.

A view of iris 12 is better seen in FIG. 2 which shows a sectional View of section 11 through section 2 2 of FIG. l. From FIGS. 1 and 2, it is seen that the section 11 includes top and bottom walls 14 and 15 and side walls `16 and 17. The particular dimensions of the irises are determined to provide the desired filtering l characteristics by techniques described in the publications cited above or any other suitable method.

The following analysis will be helpful in understanding how separating the reflecting elements by an integral multiple of half guide wavelengths, where the integral multiple is at least two, preserves .the selectivity characteristics while increasing the power handling capabilities. It is convenient to designate current and voltage relations in the vicinity of the reflecting elements at a fixed time with the subscripts L and S designating the load and source side of the elements, respectively. Assuming that the power flows from Ileft to right in the structure of FIG. 1 4and labeling parameters in the Vicinity of irises 13 and 12 with subscripts 1 and 2 respectively, the following equations relate the currents, i, to the volt-ages, v.

where i p-J S n M and r is a positive real factor corresponding to the coupling reactance X2 referred to in the portion of vol. 9 of the M.I.T. Radiation Laboratory Series cited above. L/go is substantially lone-half at the center frequency of the lter including the section 11 when K=1, L being the distance between reflecting elements 12 and 13. The guide wavelength `at center frequency is designated Ago.

Solution of the above equations may be effected by matrix manipulation to express vgS/i-ZS as a function of VIL/im. This relationship establishes the frequency characteristics of `filter section 11 and is found by performing the following multiplication of matrices.

Wl tml Wl j/t/Eo p 1 NEG It is convenient to designate these three matrices -as R2, W21 and R1, respectively, corresponding to the characteristics of refiecting element 12, the waveguide section coupling reflecting element 12 yto reflecting element 13 and reflecting element 13, respectively. The frequency-sensitive characteristics of section 11 are then fully characterized by [Ri'WziRzl This matrix product is p -j sin and L is k times the guide half wavelength at center frequency; for small values rof p, the sine may be approximated by the difference between an appropriate integral multiple of 11- and its argument. Thus,

In conventional filters, k=l.

It can be shown that increasing the length of section 11 by a factor k correspond-s to substituting kp for p in the matrix representations of characteristics of the filter section. The Itransfer characteristics of the section are then represented by the product matrix [R2] [W21]k[R1].

To show an example illustrating the propriety of replacing p by kp when k half wavelength sections are between adjacent irises, consider `the case where k=-2.

Then [w21]2 is given by [il] l? If p is replaced by a factor kp, the magnitude of the functional relationships characterized by this matrix remains unchanged if r1 and r2 are each decreased by a factor k. The matrix product remains The preceding discussion should facilitate understanding the description which follows of a method for maliwhich reduces to ing lhigh power filters. First, the number and value of the rs are determined by a suitable synthesis method. 'When these coefiicients are determined, irises or other suitable reflecting elements are designed by well-'known analytical Iand/or experimental techniques to provide corresponding values for the VSWR at the plane in the waveguide where each is located. r.The square of the maximum voltage in the cavities between reflecting elements spaced by a half wavelength may then be determined by analytical, graphical or experimental means. From a `knowledge of the prescribed power handling capabilities of the filter, it may be determined in which cavities the maximum voltage squared should be reduced in order to avoid breakdown. The elements sep- -arated by these cavities are then moved one or more additional half wavelengths apart. The increased sep- -aration between only some elements will not appreciably affect the frequency response characteristics of preceding sections. This will be better understood from the fol-- lowing considerations.

The relation between the current on the source side of iris 13 in section 11 to develop a given voltage on the load side is Thus, the current on the source side of iris 13 has been reduced by a factor of vk. Itis well known in the microwave art that a current maximum occurs at an inductive iris and that a voltage maximum occurs substantially a quarter wavelength away from the current maximum and is proportional to the value of the current maximum. Therefore, the maximum voltage in the cavity is reduced by the same factor \/k. Since the peak power is proportional to the square of the maximum voltage, it follows that the power transmitted through the section may be increased by a factor k in a k half wavelength section over that which may be transmitted through a section only a half wavelength long before the breakdown potential of the cavity is exceeded.

With reference to FIG. 3, there is shown a lengthwise sectional view of a lter having four cavities 3134 separating ive inductive irises 41-45 by one and onehalf guide wavelengths at the filter center frequency. The square of the maximum voltage of each cavity was experimentally determined, the irises being dimensioned to provide a filter exhibiting Tchebyshev characteristics within the pass band. These measurements are graphically represented on a common frequency scale about the filter center frequency in FIG. 4 with the amplitude normalized. The curves are labelled with the number of the cavity having the plotted maximum Voltage squared.

Examination of these curves shows that the two inner cavities 32 and 33 have a maximum voltage squared below two over the frequency band from 9450 to 959@ megacycles while the two end cavities 31 and 34 have a maximum voltage squared below one over the entire range. lf it is desired to provide a filter operative over this frequency band of minimum length to transmit a prescribed power, the inner sections should be twice as long as the end sections.

If inner cavities a guide wavelength long are capable of transmitting the desired power levels, then the end cavities need only be a half guide wavelength. A lengthwise sectional view of such a filter is shown in FlG. 5 with elements designated by the reference numeral primed of a corresponding element shown in FIG. 3. This embodiment of the invention provides the same degree of selectively as the filter of PEG. 3 but minimizes the size of the filter in view of the power level requirements.

The information in FIG. 4 is also helpful in determining the proper lengths of sections when operation over a wider bandwidth is required. For example, consider operation over `a bandwidth from 9425 to 9525 megacycles. The maximum voltages squared of the cavities 31-34 are below one, 2.2, 3.7 and 3.3, respectively. The ratio of lengths would then be 1:2.5:4:4. ln terms of guide half wavelengths, the integer k for the cavities in order from the source end to the load end would be 2, 5, 8 and 8. In general, the pattern for selecting the cavity lengths in the optimum manner involves choosing the length of the cavity having the highest maximum voltage squared within the desired band to be the smallest integral multiple of a half guide wavelength which results in transmission of the desired power levels without exceeding the breakdown potential of the cavity. The remaining cavities are then made a lesser or equal integral number of half guide wavelengths long, the particular integer also being the minimum which results in transmission of the desired power levels without exceeding the breakdown potentials of the respective cavities.

In a representative embodiment of the type illustrated in FIG. 5, wherein the waveguide section was 2.85 cm. wide by 1.25 cm. across the narrow dimension, the length of inner cavities 32 and 33 was 3.75 cm. and that of end cavities 31 and 34', 1.875 cm. The width of the centered conducting plates defining inner inductive irises 42', 43 and 44 was 1.6 cm. and the width of the centered conducting plates defining inductive end irises 41 and 45 was 0.8 cm. This filter selectively transmitted 80 peak kilowatts of power, its bandwidth being 60 megacycles between the one db points.

lt is apparent that those skilled in the art may now make numerous departures from the specic embodiments and techniques described herein without departing from the inventive concepts. Consequently, the invention is to be construed as limited only by the spirit and scope of the appended claims.

What is claimed is:

1. A direct-coupled resonant cavity microwave lter responsive to microwave energy having spectral components near a predetermined center microwave frequency comprising, a waveguide section of uniform cross-section having a cutoff frequency below the frequency of said spectral components, and a plurality of reective reactive elements in respective planes within said section, said planes being spaced by an integral multiple of the guide half wavelength of said energy at said center frequency, at least two of the intermediate adjacent retiective elements being separated by at least one guide wavelength of said energy at said center frequency.

2. A direct-coupled resonant cavity microwave filter responsive to microwave energy having spectral components near a predetermined center microwave frequency comprising, a waveguide section of uniform cross-section having a cutoff frequency below the frequency of said spectral components, and a plurality of irises in respective planes within said section, said planes being spaced by an integral multiple of the guide half wavelength of said energy at said center frequency, at least two intermediate adjacent planes being separated by at least one guide wavelength of said energy at said center frequency.

3. A direct-coupled resonant cavity microwave lter responsive to microwave energy having spectral components near a predetermined center microwave frequency comprising, a plurality of cascaded sections of waveguide dimensioned to normally propagate said energy, all of said sections being uniform in cross-section retiecting elements in said waveguide between adjacent sections coacting with said sections to selectively transmit only a narrow band of said spectral components, each of said sections having a length substantially equal to an integral multiple of half the guide wavelength at said center frequency, at least one of the intermediate sections being at least one guide wavelength long.

4. A direct-coupled resonant cavity microwave filter responsive to microwave energy having spectral components near a predetermined center microwave frequency comprising, a plurality of cascaded sections of waveguide having a uniform cross-section and being dimensioned to normally propagate said energy, reecting elements in said waveguide between adjacent sections coacting with said sections to provide a filter exhibiting a predetermined selectivity characteristic whereby only a narrow band of said spectral components is transmitted therethrough, each of said sections having a length substantially equal to an integral multiple k of half the guide wavelength at said center frequency, k being at least two for an intermediate one of said sections, the reflection coeicient of those of said reflecting elements adjacent to the ends of said one section being reduced by said factor k over the value required to provide said predetermined selectivity characteristic with said one section being only half said guide wavelength long.

5. A direct-coupled resonant cavity microwave lter responsive to microwave energy having spectral components near a predetermined center microwave frequency comprising, a plurality of cascaded sections of waveguide having a uniform cross-section and being dimensioned to normally propagate said energy, reflecting elements in said waveguide between adjacent sections coacting with said sections to provide a filter exhibiting a predetermined selectivity characteristic whereby only a narrow band of said spectral components is transmitted therethrough, each of said sections having a length substantially equal to that integral multiple k of half the guide Wavelength at said center frequency resulting in approximately equal values of the square of maximum voltage being developed in each section, k being at least two for one of the intermediate sections, the reection coecient of those of said retlecting elements adjacent to ends of each section being reduced by the associated factor k over the value required to provide said predetermined selectivity characteristic with all said sections being only half said guide wavelength long.

6. A direct-coupled resonant cavity microwave filter for selectively transmitting a narrow band of frequencies about a predetermined center frequency at high power levels comprising, a plurality of cascaded reliecting elements, respective waveguide sections of uniform cross-section separating adjacent ones of said reecting elements and coacting therewith to provide a predetermined frequency selectivity, said reecting elements projecting into the waveguide formed by said cascaded sections, each of said sections being dimensioned to normally propagate microwave energy within said narrow band of frequencies and having a length substantially equal to half the guide wavelength at said center frequency multiplied by an integer k, at least one pair of intermediate adjacent reflecting elements being separated by a long section of waveguide having a length such that said integer k is greater than one, the reflection coeiiicients of said one pair of reecting elements being reduced by the multiplicative integer k of said long section over the value required to provide said predetermined frequency selectivity with said long section being only half said guide wavelength long.

7. A microwave filter in accordance with claim 6 wherein each section which would develop a maximum voltage therein greater than its breakdown potential when transmitting said high power levels if only half said guide wavelength long is the minimum value of k guide half wavelengths long required to reduce said maximum voltage to a value below said breakdown potential when transmitting said high power levels while said filter exhibits said predetermined selectivity.

8. A microwave iilter in accordance with claim 7 wherein said retiecting elements are inductive irises.

References Cited in the le of this patent UNITED STATES PATENTS 2,546,742 Gutton et al Mar. 27, 1951 2,623,120 Zobel Dec. 23, 1952 2,859,418 Vogelman Nov. 4, 1958 

